A turbocharger for an automobile has been downsized for the purpose of improving fuel economy. Also, the exhaust gas temperature has been increased for the purpose of improving performance.
In response to the need for reduced size and improved performance, there is proposed a turbine rotor including a TiAl turbine wheel having high thermal resistance and a steel shaft joined to each other by Ni brazing. For instance, Patent Document 1 (JP2000-202683), Patent Document 2 (JPH10-193087), Patent Document 3 (JPH10-118764) etc are known.
Patent Document 1 discloses a structure in which a turbine wheel of TiAl intermetallic-based alloy and a carbon steel shaft are joined to each other via an intermediate material. The turbine wheel is joined to the intermediate material so that the projection-like joint portion of the turbine wheel is fit into the recess-like joint portion of the intermediate material and a brazing filler metal is inserted therein.
Patent Document 2 also discloses a structure in which a TiAl turbine wheel and a structural or martensite heat-resistant steel rotor shaft are joined to each other by inserting a brazing filler metal (silver brazing, nickel brazing, or copper brazing) between a projection of the TiAl turbine wheel and a recess of the shaft so as to fit the projection into the recess. Similarly, Patent Document 3 discloses a structure in which a TiAl turbine wheel and a structural or martensite heat-resistant steel rotor shaft are joined to each other via a brazing filler metal.